The decay of triplet pyrazine and pyrazine-D₄ in supersonic jets: Isotope effects

We have measured the intersystem crossing (ISC) rates of optically excited triplet pyrazine-d₄ in supersonic expansion. ISC rates ranging from 3.3×10² to 1.3×10³ were found in the 1154 cm^-1 range of excess vibrational energy above the T₁ origin. These rates were substantially lower than the ISC rate of pyrazine. Our measurements were accompanied by model calculations of the ISC rates of pyrazine and pyrazine-d₄. According to this model, certain vibrational modes, which undergo large frequency reduction in the excited state, regulate the strong vibrational energy dependence of the T₁→S₀ ISC of pyrazine. The large frequency changes result from the interaction of the near lying ³nπ* and ³ππ* states (the "proximity effect"). These calculations account quantitatively for the excess vibrational energy dependence of the ISC rates in pyrazine, as well as for the isotopic substitutional effect.